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Palmer Amaranth Flexing Its Muscle

Monday, February 1, 2010
filed under: Weeds

Let there be no doubt: Palmer amaranth has become an increasingly “growing” weed problem in Kansas sunflower fields.

If you question that statement, consider the results of the annual National Sunflower Association crop survey. This coordinated survey — conducted each September and encompassing several key sunflower states and the Canadian province of Manitoba — has found Palmer amaranth to be the weed of highest incidence in Kansas sunflower fields for the past five years.

In 2005, Palmer amaranth populations were found in about 18% of the surveyed Kansas fields, with the next most common weed, puncture vine, observed in 9% of the fields. The next year, 2006, the incidence level for Palmer amaranth exploded to 66% of surveyed Kansas fields, while that of puncture vine crept up to only about 12%. The 2007 survey found Palmer amaranth in 88% of Kansas fields surveyed, with puncture vine jumping all the way to 75%. The following year, 2008, saw the Palmer amaranth incidence level climb to 92%, while puncture vine dropped to 64%. The 2009 NSA crop survey found Palmer amaranth in 90% of the surveyed Kansas sunflower fields, wile puncture vine was noted in just 20%.

Phil Stahlman is not the least bit surprised by these findings. “Palmer amaranth has become the most common of all the pigweed species in the southern and central Great Plains,” says the Kansas State University weed scientist. Stahlman, who is based at KSU’s Hays Agricultural Research Station, says he’s viewed Palmer amaranth plants flowering when just two inches tall. He’s also seen plants of this weed species growing to a seven- or eight-foot height.

Obviously aggressive, Palmer amaranth can be a serious competitor for sunflower, and numerous High Plains growers can attest to its impact on seed yields. In one trial where Stahlman compared hand-weeded plots to others with heavy Palmer amaranth pressure, this weed’s presence slashed sunflower seed yields by 54%.

Palmer amaranth also threatens a crop like sunflower because it germinates over an extended period of time. “Even in midsummer, if conditions are right,” Stahlman observes. It thus can be an issue for double-cropped sunflower fields as well as full-season ones, although the weed pressure typically is lower in second-crop ’flowers.

What are the sunflower grower’s best options for controlling this increasingly prolific weed?

The program should begin in the rotation’s preceding crops. “We probably have the best success following a Roundup Ready® crop, simply because of the effectiveness of the [glyphosate] in that crop,” Stahlman says. If following wheat, as do a lot of Kansas ’flowers, a fall burndown with glyphosate or another product will set the stage for good control in the next year’s sunflower.

In-season control of Palmer amaranth can be achieved with Beyond® or Express® if the sunflower hybrid being grown is a Clearfield® or ExpressSun® variety. If, however, the Palmer amaranth population should have ALS resistance (as is the case in a number of fields throughout Kansas), these products will not do the job. Beyond will have more residual control than Express, the KSU weed scientist adds — something to consider, given Palmer amaranth’s long germination window.

As for the available preplant or preemerge herbicides, Spartan® is most consistent in performance and provides the best degree of control of Palmer amaranth, Stahlman observes. Dual® and Prowl® will provide partial control under the right conditions, but don’t have the residual effect that Spartan does, he remarks. “Spartan is probably our preferred broadleaf weed control herbicide,” Stahlman says. “But some growers are still using quite a bit of Prowl, and some are using Dual. It depends on the weed spectrum they’re anticipating.

“We do need to stay on top of Palmer amaranth in sunflower, because it is such a highly competitive weed,” emphasizes the Kansas weed scientist. — Don Lilleboe

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